A Mach-Zehnder modulator (MZM) conventionally functions as a "high-fidelity" electrical-to-optical modulator--i.e., as a device for converting electrical signals to optical signals. Ordinarily, an MZM is operated at a bias so that an input voltage signal applied to the MZM modulates the amplitude of a continuous wave (CW) laser beam passing through the MZM. The amplitude-modulated laser beam is then detected by a photodetector, which produces a "high-fidelity" photocurrent signal (i.e., an electrical current output signal) that is substantially distortionless with respect to the input voltage signal.
The input voltage signal applied to the MZM to modulate the amplitude of the CW laser beam is typically a radiofrequency (RF) signal. The output signal from the photodetector (i.e., the photocurrent signal) is said to be distortionless with respect to the RF input signal--i.e., the MZM functions as a "high-fidelity" electrical-to-optical modulator--when the waveform of the photocurrent signal is the same as the waveform of the RF input signal. High-fidelity operation of the MZM is achieved by biasing the MZM to operate in a linear range in which the photocurrent signal varies substantially linearly with respect to the RF input signal.
It had not been recognized in the prior art that an MZM (or any other type of interferometric modulator) operating outside the linear range would have practical utility. Specifically, it was not realized in the prior art that an interferometric modulator such as an MZM operating outside the linear range could be used to advantage for photonically mixing electrical signals.